The UK's coal-fired power stations take too long to get started to have them turned on and off on a regular basis. They run all the time, day and night. This is fine during the day but much less electricity is used a night. They generate power that is not used. There are several places in the country where the surplus electricity is used to pump water up hill. The water gains gravitational potential energy (mgh). The energy is stored until there is a shortage. The barriers are opened and the water flows back down hill through turbines that generate electricity again. The example photographed is on Loch Ness. The middle photograph shows the turbine shed in the distance. The water is pumped to Loch Mhor which is several miles away. The information says that there are two 150MW generators run by the turbines. It says that are peak flow, 200 tonnes of water flows every second. Use mgh to work out the gravitational potential energy lost in one second - this is the power input to the turbines. (You can get the heights by enlarging the bottom picture). Then work out the efficiency of the two turbine generators combined. (1 tonne = 1000kg. MW means Mega Watts which is 1 million Watts)
